I'm rereading SmF right now and that's exactly what I was wondering about: Why did Mab bother? Ok, her Accords were broken. But so what, someone who can't defend themselves doesn't have any business joining the club.
What value is the accords if the only enforcement is the victim defending themselves? That is the definition of a toothless agreement.
The value in the accords is that community of supernatural agents enforces a code of conduct. In other words, people who break the laws are punished. Mab created the accord for a reason, so she has a good reason to act to punish those who break the accord.
If that is not good enough, the second reason is just as good. The accords are an agreement - a bargain. Mab keeps her word and the agreement binds her to punish those who break it. That is winter law.
And if that is not good enough, the third reasons is helpful. Mab is limited in her ability to act. One of the "loopholes" in these limits are the bargains Mab makes. By breaking the accords, Mab is freed to act. She must certainly act in accordance to her obligations in the accords, but that still leaves her a lot of flexibility in how she chooses to act. For example, in this example she gained additional benefits in training Harry, earned a debt from Marcone, enhanced her reputation, and blocked the actions of a denarian who was almost certainly involved in the assault on Arctus Tor. That's a lot of benefit should would not have been able to achieve without the breach in the accords.